The present invention relates to viscous fluid couplings, and more particularly, to such couplings which are capable of more than one ratio of output speed to prime mover speed.
It will be apparent from a reading of the specification that the present invention may be advantageously utilized with fluid couplings intended for many different applications. However, the invention is especially useful when applied to a viscous fluid coupling which serves as a drive for the radiator cooling fan of a vehicle engine, and will be described in connection therewith.
Those working in the viscous fan drive art have long recognized the desirability of a fan drive having a high speed ratio (i.e., ratio of output or fan speed to input or engine speed) at low engine speeds when the least air is being forced through the radiator merely by movement of the vehicle.
It has also been recognized that as engine speed increases, it is desirable to decrease the speed ratio so that less horsepower is being wasted driving the fan when the higher vehicle speed is forcing a greater amount of air through the radiator. Reducing this speed ratio of the fan drive at higher engine speeds is also desirable to reduce the heat buildup within the fan drive.
Recognition of the need for a fan drive having more than one speed ratio is evidenced by U.S. Pat. No. 3,444,748, assigned to the assignee of the present invention, which illustrates a fan drive including a fluid coupling providing a higher ratio at lower engine speeds, and an overrunning clutch providing lower ratios at high engine speeds. However, the fan drive shown in the cited patent includes no provision for disengagement when the air temperature within the engine compartment is such that operation of the fan is unnecessary. Furthermore, the fan drive of the cited patent could not be modified in an obvious manner to provide for temperature-responsive disengagement at higher engine speeds, at which there is a direct mechanical connection or drive from the input to the fan through the overrunning clutch.